The so-called Rim Fire has charred 74,843 hectares - an area larger than the land mass of Chicago - since it erupted on August 17.

Most of that has occurred in the Stanislaus National Forest west of Yosemite, according to Mr Berlant.

It now ranks as the largest California wildfire since October 2007, when the Witch Fire torched more than 1,600 structures in San Diego County, and one of the 10 biggest in state history, according to CalFire records.

A main road into Yosemite, one of America's top tourist destinations, remains closed, while two famous stands of giant sequoia trees, the Merced Grove and the Tuolumne Grove, were in the path of advancing flames.

But the vast majority of the 1,200-square-mile park, including the Yosemite Valley area renowned for its towering rock formations, waterfalls, meadows and pine forests, remains open to the public.

The Rim Fire, named for a Stanislaus National Forest lookout point called Rim of the World, has already damaged two of the three hydropower generating stations linked to the Hetch Hetchy Reservoir that supply electricity for all of San Francisco's public facilities, such as hospitals and firehouses.

But the city has been drawing on reserve power stored for emergencies and purchasing additional electricity on the open market to make up for the difference.